Transmission of information by the use of light over optical fibers is widely used in long-haul telecommunication systems. Optical signals are generated, transported along optical fibers and detected to regenerate the original electronic signal with as little change as possible. Fibers are substituted for other transmission media and all signal processing is done electronically, resulting in lowered cost and high quality digital transmission.
As fiber optic applications technology develops direct optical processing of signals without conversion to electronic signals will be required. Optical fiber systems will be applied in computer networks, for example, in multiple access computer networks. Such applications will require optical fiber devices such as amplifiers, multiplex/demultiplexers, splitters, couplers, filters, equalizers, switches and other optical signal processors.
An economical low-loss, easily and reproducibly manufactured single-mode optical fiber filter, the design of which can be adapted to a desired bandwidth, FSR and finesse is an important component for such fiber optic systems. A fiber Fabry-Perot (FFP) interferometric filter is such a component.
The Fabry-Perot (FP) Interferometer was first described by C. Fabry and A. Perot in 1897 (Ann. Chem. Phys., 12:459-501) and has since found wide use in a variety of applications of optical filters. The basic structure and operation of the FP interferometer is well-known in the art and is described in many physics and optics texts (see, for example, E. Hecht "Optics" 2nd. Edition (1987) Addison-Wesley, Reading MA, p. 369). This interferometer consists of an optical cavity formed between two typically highly-reflective, low-loss, partially transmitting mirrors. Lenses are typically used to collimate divergent optical beams for processing through the FP interferometer.
While single-mode optical fibers can be used with lensed conventional FP interferometers, lenses with large beam expansion ratios are required and result in reduced stability and poor optical performance. The adaptation of FP cavities for optical fiber filters had been hindered by the lack of practical designs for FFPs with appropriate optical properties. Recently, FFPs which possess optical properties suitable for telecommunication applications have been described. These FFPs consist of two highly-reflective, preferably plane-parallel mirrors, forming the optical cavity through at least a portion of which, in most cases, a length of single-mode optical fiber extends. This basic design eliminates the need for collimating and focusing lenses, improves stability and optical performance and makes the FFPs compatible with single-mode optical fibers and other fiber devices.
In 1987, J. Stone and L. W. Stulz described three configurations of FFP interferometric filters (Elect. Lett., 23(15):781-783, 1987) that span a wide spectrum of bandwidths and tuning ranges. The Type I FFP is a long cavity FFP in which mirrors are deposited at the ends of a continuous fiber. The minimal cavity length is about 1 cm (FSR of about 10 GHz), so that this long cavity device is not necessarily important for telecommunication applications. In the Type I FFP, the fiber can be stretched by piezoelectric transducers (PZTs) to produce tuning of the bandwidth (BW) over the free spectral range (FSR).
The Type II FFP of Stone and Stulz is a gap resonator which has no optical fiber inside the optical cavity and so can exhibit significant losses. Due to such losses, the useful cavity length of this type of FFP is less than about 5 .mu.m. The Type II FFP is also not well-suited for telecommunication applications.
The Type III FFP is better suited to telecommunication applications than the Type I and VI FFPs. This type of FFP has an internal waveguide interposed between external fiber ends. Mirrors are positioned at an external fiber end and at one end of the waveguide. The waveguide is comprised within the optical cavity. The optical cavity also contains a fiber gap, for example between the waveguide and one of the external fiber ends, the width of which is fixed or can be changed to tune the filter.
The ferrule components and waveguide of Type II and III FFPs must be axially aligned to high precision in order to minimize transmission loss. Type VI and III FFPs are the subject of U.S. Pat. No. 4,861,136. This patent relates to FFPs which are tuned by use of PZTs to change the cavity length. In order to use PZTs to change resonance cavity length without detriment to alignment, elaborate alignment brackets and fixtures are necessary.
U.S. Pat. No. 5,062,684 describes an improved tunable FFP filter in which the resonance cavity is formed by two wafered ferrules with mirrors embedded between the wafer and the ferrule and axially disposed optical fibers. The two ferrules are positioned in the filter configuration with mirrors opposed and the optical fibers of the ferrules aligned. The resonance cavity formed between the embedded mirrors contains a fiber gap between the wafered ends of the ferrules. The ferrule combination is held in alignment by an alignment fixture including piezoelectric transducers which function to change the resonance cavity length on application of a voltage to the transducer. A support fixture useful for holding a FFP ferrule assembly in axial alignment is described in EP patent application 0 457 484. This fixture also provides a means for minor adjustment of alignment as well an electronic means employing PZTs transducers for changing the cavity length.
A major problem of FFP filters is signal loss due to wavelength drift as a function of the change in cavity length of the filter with temperature. An uncompensated FFP, like that of U.S. Pat. No. 5,062,684 or EP application 0 457 484, can exhibit a relatively large change in cavity length, of the order 0.05 .mu.m/.degree.C. This can represent a drift of a full FSR (free spectral range) over 15.degree. C. See C. M. Miller and F. J. Janniello (1990) Electronics Letters 26:2122-2123. Control circuitry has been employed with PZT-tuned FFPs to lock the filter onto a wavelength over a wide temperature range (I. P. Kaminow (1987) Electronics Letters 23:1102-1103 and D. A. Fishman et al. (1990) Photonics Technology Letters pp. 662-664). In these systems, control voltage swings of several tens of volts were required to compensate for the relatively large change in cavity length with temperature. Wavelength locking of FFP filters can prevent signal loss, however since approximately 20 volts applied to a PZT is needed to tune through an FSR, a total power supply range of about 60 volts is needed to maintain the wavelength lock over an operationally useful temperature range of about 30.degree. C. (Fishman et al. supra).
Miller and Janniello (1990) supra described passive temperature compensation of PZT-tuned FFPs. Since PZTs require a higher voltage at higher temperature to maintain a given length, cavity length effectively decreases with increasing temperature (with constant voltage). Thus, the PZT-tuned FFP has a negative temperature coefficient. Addition of a material having a positive temperature coefficient in series with the PZTs, for example aluminum blocks, was found to compensate for the negative temperature coefficient of the PZTs. This method of passive compensation significantly reduced the voltage requirements for FFP locking circuits such that +/-12 volt power supplies, such as are conventionally employed in computer systems, could be employed for locking.
The manufacturing yield of highly accurate, passively compensated FFPs is low. This is due in part to the fact that it is difficult to obtain the required degree of passive temperature compensation in any particular filter. The filter assembly must be entirely constructed before the extent of compensation achieved can be tested. With such passive compensation there is no means for adjusting the temperature coefficient of the filter after the filter has been fabricated. FFPs are often over or under compensated.
The present invention provides FFPs which overcome the difficulties described above. In the filters of this invention, the temperature coefficient can be selectively varied after fabrication of the filter in order to minimize the variation of the cavity length as a function of temperature. The filter support fixtures and methods described herein provide FFPs which on average have a significantly lower temperature coefficient than previously described temperature compensated FFPs. Application of the methods herein for filter fabrication and the use of the support structures herein result in significantly improved manufacturing yield of FFPs having desirable optical characteristics and low thermal coefficients.